So I was messing around on the PTR and decided to make a guild... A very large guild. I used ninjainvite/rapedotty to start invites. When I hit around 400-500 members I started getting a bunch of client crashes. At first I thought it was the addon, but soon people were telling me they were getting crashes too. It seems that about half the time you log in, guild chat won't work (can't read it, can't talk in it) and the game runs fine. If however, you log in and gchat happens to be working, you will get a client crash shortly after seeing the chat or trying to talk in it. The guild is over 1200 members now, and I am getting a ton of whispers "your guild crashes my client". I am only able to stay online when the gchat is broken, but the new invites see the chat and often crash shortly after joining the guild. Not an extremely useful exploit, but its fun to see 20-30 people from guild go offline all at once.
edit: some people are claiming that they are unable to even log on after joining my guild, their client crashes before they can type "/gquit"